Add comment to better clarify instance constraints addition in IsCmd

src/Gargantext/Database/Class.hs

@@ -62,6 +62,10 @@ type IsCmd env err m =
   ( MonadReader     env     m
   , MonadError          err m
   , MonadBaseControl IO     m
+  -- These 3 instances below are needed because in the transactional code
+  -- we can throw 'err' as an exception, which requires 'err' to be an 'Exception'
+  -- and thus have a 'Show' and 'Typeable' instances. The fact that we can catch
+  -- exceptions in the evaluator of the 'DBTx' monad code means we need a 'MonadCatch'.
   , Typeable err
   , Show err
   , MonadCatch m